.container {
  height: 100px;
  border: 1px solid #c3c3c3;
  display: flex;
}
.box_1, .box_2, .box_3 {
  flex-grow: 1;
}

.box_1 {
  background-color: coral;
}

.box_2 {
  background-color: lightskyblue;
  animation: grow 5s infinite;
}

.box_3 {
  background-color: khaki;
}

@keyframes grow {
  50% {
    flex-grow: 6;
  }
}